The present disclosure relates generally to power conversion, and particularly to transformer control.
Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits. In a transformer, an alternating current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. A group of transformers may be connected together to provide specific operating characteristics.
Imbalance of transformer flux among the group of transformers may result in the generation of a flux direct current (DC) offset. In a perfect environment, it is preferable that transformers not be driven with DC nor, generally, have any DC component present at the input. Relatively small amounts of DC can cause core saturation, current distortion, and increased losses, thereby preventing proper operation. Also, because a DC voltage source does not provide a time-varying flux in the core, an induced counter-EMF is not generated, resulting in current flow into the transformer that is limited only by the series resistance of the transformer windings. In this situation, the transformer would heat until it either reaches thermal equilibrium or is possibly damaged.
One cause of core saturation is the accumulation of low frequency flux, such as when a motor is starting, for example. Another cause of flux saturation within series connected transformers is flux imbalance among the different transformers, which may result from disturbances within the grid and variation among transformer parameters or switching patterns. Flux balance control is generally implemented by controlling the average value of transformer flux to be zero. This will involve the moving-average calculation of flux. However, during very low frequency operation, such as motor startup, the delay caused by the average flux calculation is too large, making it difficult to control the flux balance within an acceptable period. Accordingly, there is a need in the art for a transformer arrangement that overcomes these drawbacks.